This section provides background information related to the present disclosure which is not necessarily prior art.
Battery cell internal resistance is an indication of battery cell health. Battery monitoring systems often used in uninterruptible power supply (“UPS”) systems may determine individual cell internal resistance by periodically injecting a pulse of DC current through the cell and measuring the DC voltage developed across the cell during the current pulse test. This voltage is compared to the normal DC voltage across the cell to derive the cell internal resistance. However, the AC ripple voltage inherently present on an uninterruptible power supply (“UPS”) system DC bus creates error in the test voltage measurements and therefore errors in the internal resistance calculation. Voltage ripple is introduced onto the front end of a UPS DC bus by rectification circuits of the UPS which apply current pulses from an upstream power source to maintain the DC bus voltage at the desired potential for proper UPS operation and to keep the battery cells of the UPS in a fully charged state. Voltage ripple is also introduced onto the DC bus by inverter circuits of the UPS which extract current pulses from the DC bus in the process of supplying power to the UPS load. The UPS DC bus ripple voltage signals may be random and are in the approximate amplitude and frequency range of the voltage signals generated by the battery cell test. The voltage ripple signals can create a signal-to-noise ratio concern with respect to the voltage measurements being taken. This can make it virtually impossible to effectively filter the AC ripple voltage from the desired DC voltage signal generated by the battery cell test.